This disclosure relates generally to writing/drawing tablets, and in particular, to Cholesteric Liquid Crystal Displays (ChLCDs) used in such writing/drawing tablets.
The market for toys that are low-cost, low-power electronic devices that capture and display writing and drawing by hand continues to grow and is increasingly competitive. Consumers desire rugged devices that are easy to use and capture the experience of handwriting similar to the feel of writing with a pen on paper. In a world that strives for sustainability of natural resources, consumers and manufacturers seek alternatives to paper and paper-based solutions for easily and inexpensively capturing hand-written or hand-drawn ideas, information and images. Consumers also want their electronic devices to capture and present text and images in vibrant, unique and colorful ways, to mimic the use of pens and paper of different colors, without the expense, mess and single-use of pen and paper. Device ruggedness and visual appeal are particularly important for toys to appeal to children and to consumers purchasing products for children for both education and entertainment purposes.
In 2010, the Boogie Board® pressure sensitive cholesteric liquid crystal writing tablet, also referred to as an eWriter, of Kent Displays Inc. appeared on the market in which a pointed stylus or the finger can be used to write or trace an image on the surface of the tablet as described in U.S. Pat. Nos. 6,104,448 and 9,116,379, both patents being incorporated herein by reference. In a cholesteric liquid crystal writing tablet, the liquid crystal is sandwiched between two substrates that are spaced to a particular cell gap. The upper substrate is flexible and the bottom substrate is either opaque or semitransparent. Within the cell gap is a bistable cholesteric liquid crystal layer which can exhibit two textures, an essentially transparent (focal conic) texture and a color reflective (planar) texture. The spacing of the cell gap is usually set by plastic or glass spacers that are either cylindrical or spherical in shape. The tablet is initialized by applying voltage pulses to the electrodes to electrically drive the cholesteric material to the focal conic state. When one presses on the top substrate with a pointed stylus or finger, the liquid crystal is locally displaced. Flow induced in the liquid crystal changes its optical texture from essentially transparent to a brilliant reflective color at the location of the stylus. The reflective color contrasts well with the opaque or semitransparent color below the liquid crystal layer. An image traced by the stylus or finger will remain on the tablet indefinitely without application of a voltage until erased. Erasure is accomplished by applying a voltage pulse to transparent conducting electrodes on the inner surfaces of the substrates that drives the cholesteric liquid crystal from its color reflective state back to its essentially transparent state.
The above described principle is disclosed in more detail in U.S. Pat. No. 6,104,448, which is incorporated herein by reference. Another mode of operation that can be used for the erasure of a selected portion of the image is disclosed in U.S. Pat. No. 8,139,039, which is incorporated herein by reference and is applicable to the pressure sensitive liquid crystal devices of the present disclosure. This mode of operation is different from that described above. The tablet is initialized by electrically driving the pressure sensitive liquid crystal device to the color reflective texture with a voltage pulse or pulses. Then with a continuous voltage applied to the electrodes of an appropriate value, one can write images by driving the cholesteric material to the substantially transparent texture with the pressure of a pointed stylus. This mode of operation with a color reflective background is termed Mode A whereas the other mode with an essentially transparent background is termed Mode B.
The commercial Boogie Board® writing tablet, operated in Mode B, has the color black for the fixed opaque light absorbing background. The dark black background offers high contrast for the color reflective image written on the tablet. As disclosed in U.S. Pat. No. 5,493,430, which is incorporated herein by reference, other opaque colors may also be used for the fixed background of a ChLCD. The color of the background additively mixes with the color reflected by the cholesteric liquid crystal to present a different color than that of the cholesteric liquid crystal. There may be multiple colors on the background and those colors may be patterned. As an example, the pattern could be lines offering a lined tablet for convenience in writing text similar to a ruled paper tablet as described in U.S. Patent Application Publication No. 2013/0314621, which is incorporated herein by reference.
Device Features and Operation of Current eWriters by Kent Displays Inc.
Referring to FIGS. 1A, 1B, a light absorbing coating 1 is placed on the bottom transparent plastic substrate 2, which is coated on its other surface with a transparent electrically conductive layer 3. This coating 1 can also be patterned as described in U.S. Patent Application Publication No. 2013/0314621. The remainder of the pressure sensitive liquid crystal device includes a liquid crystal layer 4 including a cured dispersion of cholesteric liquid crystal, polymer and spacers, and another transparent top substrate 6 coated on an inner surface with a transparent electrically conductive layer 5. The side of substrate 6 not coated by the conductor 5 forms a writing surface W proximal to a viewer V of the device (the viewer V looking in a general direction of the arrow). The bottom and top substrates are extended to create the bottom ledge 7 and top ledge 8. The transparent conductive layers 3 and 5 are exposed on the ledges 7 and 8, respectively, for connecting to drive electronics so that a voltage or voltage pulses may be applied across the electrically conductive layers 3 and 5 of a magnitude and pulse width appropriate to initialize or erase an image or operate the pressure sensitive liquid crystal device in at least one of Mode A, Mode B and select erase as described in U.S. Pat. Nos. 8,228,301 and 8,139,039, which are incorporated herein by reference in their entireties.
The cholesteric liquid crystal in liquid crystal layer 4 can be confined or unconfined interconnecting droplets prepared by emulsification, polymerization induced phase separation, temperature induced phase separation, solvent evaporation phase separation, or by flow reducing features in between the two conductors 3 and 5. FIG. 2 shows a side view of eWriter 340 showing the liquid crystal layer 4 with a region of unreduced cell gap 290 and reduced cell gap 300 due to the pressure from the stylus 280. The conducting electrodes 260 are connected with electrical interconnects 270 to electronic write (and erase) circuitry 301 that provides suitable voltages to the conducting electrodes 260, usually in the form of a voltage pulse. Pressure of the stylus 280 is applied to create an image. The term drive electronics has a meaning that includes write circuitry that enables writing on the device and also erase circuitry that enables erasing of the device. It should be understood that when the terms write circuitry are used this circuitry can also be capable of erasing. The write circuitry is not required to achieve writing in all uses of the device, for example, it is not needed for Mode B writing.
FIG. 3 explains the function of the writing circuitry 301. FIG. 3 is an illustration of the typical and hypothetical voltage response shown as the reflectance (R) of the eWriter versus the voltage (V) of a pulse applied to the cell. There are two curves illustrated in the voltage response of FIG. 3: a solid curve 350 for the cell with an undepressed cell gap 290 and a dashed curve 360 for a reduced cell gap spacing 300 (the undepressed and reduced cell gaps being shown in FIG. 2). In both the solid 350 and the dashed 360 curves of FIG. 2, the maximum light reflectance of the eWriter is achieved by the planar texture that is indicated by Rp 370 whereas the minimum light reflectance of the eWriter is achieved by the focal conic texture that is indicated by Rfc 380. FIG. 4 shows a typical example spectrum of the bright planar state 20 and the dark weakly scattering focal conic state 21 of an eWriter made using a black absorbing layer 1 and a cholesteric liquid crystal in the liquid crystal layer 4 that is tuned to reflect light at around 550 nm. The color reflected by the cholesteric liquid crystal can be tuned using the formula
            λ      max        =                  n        _                    ϕ        ⁢                                  ⁢        H        ⁢                                  ⁢        T        ⁢                                  ⁢        P              ,where, λmax is the desired peak reflectance wavelength, n is the average refractive index of the nematic liquid crystal molecules used, φ, is the concentration of the chiral dopant molecule used to induce a uniform twist in the nematic liquid crystal so that it can Bragg reflect, and HTP is the Helical Twisting Power of the chiral dopant.
In mode A, the procedure of writing an image is to first erase all previous images by applying an erasing voltage of value Vp indicated by vertical line 390 to drive the cell initially to the planar texture with a reflectance and color 20 of FIG. 4. The value of the voltage or magnitude of a voltage pulse to do this is well known in the art of making cholesteric liquid crystal displays e.g., U.S. Pat. Nos. 5,453,863 and 5,691,795, both patents which are incorporated herein by reference. This erases the writing tablet to the reflective planar texture so that the background color of the writing tablet is the additive effect of the reflective color of the cholesteric material in the liquid crystal layer 4 with the color of the light absorbing back layer 1 of FIG. 1 and FIG. 2 as described in U.S. Pat. No. 5,493,430, or the additive effect of the reflective color of the cholesteric material in the liquid crystal layer 4 combined with the effect of a semitransparent back layer SL as described in U.S. Pat. No. 9,116,379. In order to write an image using stylus 280 a voltage, Vw, is applied by the write circuit 301. The value of Vw is indicated by vertical line 400 of FIG. 3. The voltage is applied for the duration of the stylus write; preferably a continuous AC voltage or a sequence of bipolar pulses. The voltage is applied to the patterned or unpatterned electrodes so that the entire pressure sensitive liquid crystal device area seen by the user V of the writing tablet, has an electric field applied to it. It is seen by curve 360 in FIG. 3 that a voltage Vw 400 will drive that area of the cell to the focal conic texture with a reflectance and color 21 of FIG. 4; that is, areas of the cell under the stylus where pressure is applied and cell gap is reduced. The planar texture in that pressure applied area is driven to the focal conic texture as illustrated by the dashed curve 360. In other words, where write voltage Vw intersects dotted line 360 in FIG. 3, the reflectance is low and the liquid crystal is in the essentially transparent focal conic state, which exhibits the light absorbing back layer 1 or effects of the semitransparent back layer SL. In the remainder of the cell where the cell gap is not disturbed, even though the electric field has been applied here, the material will remain in the planar texture as indicated by the solid curve 350, leaving a planar background for the focal conic writing. In other words, the liquid crystal material where the cell gap is undisturbed remains in the planar texture as shown where line Vw intersects the solid line 350 (undepressed cell gap) while the voltage Vw is applied to the electrodes and is not converted to the focal conic texture. The write circuit 301 can also provide an erasing voltage Vp to erase the entire tablet. It is seen by FIG. 3 that a voltage Vp 390 will drive the entire cell to the planar texture, erasing the focal conic writing. The voltage may be an AC voltage pulse or a sequence of pulses, preferably bipolar pulses.
FIG. 5 shows an illustration of a prior art eWriter with interconnects 270 and drive electronics 301 operating in mode A, where 53 is the dark focal conic line after the stylus 280 has passed over it and a voltage Vw (400) is applied. The reflectance and color of this line is 21 of FIGS. 4 and 52 is the bright planar background with a reflectance and color 20 of FIG. 4.
FIG. 6 illustrates mode B. The same prior art eWriter is initially switched to the weakly scattering focal conic state 56 by applying a voltage Vf (410). The cholesteric liquid crystal in the focal conic state 56 has a reflectance and color 21 of FIG. 4. The pressure from the stylus 280, without further application of voltage, will induce flow that will cause the liquid crystal to reorient to the bright planar texture 55 with a reflectance and color 20 of FIG. 4. The liquid crystal of the viewing area can be switched back to the focal conic texture by applying voltage 410 again.
In the prior art eWriter the color and reflectivity that are exhibited by the liquid crystal layer 4 of FIG. 1 do not vary by region or location on the writing surface as illustrated by the uniformity of the planar texture 52 and 55 of FIGS. 5 and 6, respectively. The focal conic texture is shown by 53 and 56 in FIG. 5 and FIG. 6. If one were to measure the reflection spectra of the eWriter at any given region or location on surface 6 of FIG. 1 of the prior art eWriter in the bright planar texture the spectra will be of uniform shape as illustrated in 20 of FIG. 4 if the cholesteric liquid crystal is tuned to reflect at a wavelength of about 555 nm, while the focal conic state will show a spectra similar to 21 of FIG. 4. In addition, the electrooptic response of the whole prior art eWriter behaves as shown in FIG. 3; that is, the voltages used to achieve the bright and dark states do not vary by the location on the writing surface. Further, the characteristic linewidth does not vary by location on the eWriter writing surface 6.
Toys that are electronic writing and drawing devices offer the benefits of multiple uses, neatness (i.e., no cleanup of paint, broken crayons, ink stains or pencil shavings) and ruggedness. Consumers currently choose toys that are electronic devices for a wide variety of features and, until very recently, benefits related to handwriting and drawing by hand have not been a major factor. In addition, consumers seeking a toy that is a low cost, rugged device to provide to their children for entertainment—such as on long automobile trips—are often frustrated and overwhelmed by the cost, complexity and fragility of electronic devices. Many electronic devices can be easily damaged or mishandled by children in their exuberance of use, or require extensive preparation by responsible adults to limit inappropriate access by children using the device. Many toys that mimic the look of more expensive devices but without their features quickly bore the child user.
There are toy products currently on the market—such as Doodle Sketch (newer version of Magna Doodle) and Etch-a-Sketch®, both produced by the Ohio Arts Company—that allow children and other users to draw images on the device and to erase these images. The Doodle Sketch requires a specialized stylus (attached by a tether to the device) using a magnetophoretic display. The basic Etch-a-Sketch uses a built in specialized stylus controlled by two knobs to remove aluminum powder from the inside surface of a glass screen. The basic Etch-a-Sketch is limited to a single color on a single color. A Color Etch-a-Sketch was produced but this device required the user to install a colored ink marker into a stylus holder, then to lay a sheet of paper on top, then to use the knobs to write on the inner, or bottom, surface of the paper. To use a different color required removing and replacing the marker with a marker of another color. These toy products are bulky, and the toys require a specialized stylus attached to the device. These toys cannot allow the user to freely create multiple images with the natural feel of writing on paper in brilliant color with a single movement of a non-specialized stylus, including a fingernail.
U.S. Pat. No. 7,521,112 by J. Li et. al. discloses that material has to be applied to reveal what is hidden behind a removed opaque coating. In U.S. Patent Application Publication No. 2010/0247223, H. O. Ribi describes specific writing instruments and substrate pairs to create multi colored writings and patterns. In U.S. Pat. No. 5,270,087, a dark opaque wax coating is removed to reveal an image that is hidden behind the coating; this image is semi-transparent. In this case there is a messy waxy residue that is produced and the product can only be used once.
In U.S. Pat. No. 5,668,614, L.-C. Chien and J. W. Doane describe how to create a display with predefined regions of different colors laid out in a specific pattern (repeating columns of Red, Green and Blue) in a high resolution so as to create a display that can show a full color. There is no disclosure of writing on this display.
There is clearly a need for a better technology to allow consumers of all ages to easily and repeatedly write, draw and immediately/simultaneously display to share contrasting, vibrant multiple color text and images for creative, educational and therapeutic uses.
Accordingly, it is desirable to provide a portable electronic device with a pressure-sensitive rugged surface that users can write or draw upon in differing colors with any stylus, including a finger nail, to create colorful results. In addition, the present disclosure offers other desirable features and characteristics that are detailed in the subsequent Technical Summary, Detailed Description, the accompanying drawings and the appended claims.